sedimentary
Banded Calcite (often sold as 'Mexican Onyx')
Calcite (Calcium Carbonate, CaCO3)
Hardness: 3 on Mohs scale; Color: Cream, honey, brown, and green banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral. It is translucent and reacts with dilute hydrochloric acid.
- Hardness
- 3 on Mohs scale
- Color
- Cream, honey, brown, and green banding
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 3 on Mohs scale; Color: Cream, honey, brown, and green banding; Luster: Vitreous to waxy; Crystal structure: Trigonal; Cleavage: Perfect rhombohedral. It is translucent and reacts with dilute hydrochloric acid.
Formation & geological history
Formed by the precipitation of calcium carbonate from cold water solutions in caves (as flowstone or stalagmites) or from springs (travertine). It is a chemical sedimentary rock often associated with limestone regions.
Uses & applications
Primarily used for decorative purposes including carved ornaments, bowls, bookends, and jewelry. It is also used as a specialty architectural stone for countertops or tiles, though it is soft and susceptible to scratching and acid damage.
Geological facts
Despite being commonly marketed as 'Onyx', this material is actually a form of calcite or travertine. True onyx is a variety of chalcedony (silica) with a Mohs hardness of 7, making it much harder than this specimen.
Field identification & locations
Field identification can be done using a hardness test (it is easily scratched by a copper penny or steel knife) or an acid test (it will fizz when in contact with vinegar or acid). Common locations include Mexico, Pakistan, and various limestone-rich regions worldwide.
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